Generally, transformers are pieces of electrical equipment which change a voltage to a higher or lower voltage. The transformers are classified into oil-immersed transformers and dry-type transformers according to the kind of insulating material. An oil-immersed transformer filled with insulating oil is widely used. The oil-immersed transformer includes a high-voltage winding, a low-voltage winding, an iron core, insulating oil, a tank, and other components.
The oil-immersed transformer is constructed so that electric current is supplied through a bushing mounted to a bushing turret. When a breakdown occurs in the transformer due to abnormal voltage caused by lightning or a switching surge, and thus an arc is generated, some of the insulating oil filled in the tank for insulating or cooling the transformer is instantaneously burnt. Due to the combustion of the insulating oil, the internal pressure in the transformer is suddenly increased. Such pressure ruptures the transformer tank, and air fed through the ruptured portion is supplied to an arc generating part, so that a fire may break out. Further, the insulating oil escapes out of the ruptured tank, thus causing environmental pollution.
In order to prevent the tank from rupturing, the conventional method of interrupting the supply of electricity to the transformer has been widely used. However, the tank may rupture even due to the rise in pressure occurring prior to interrupting the electricity supply, and thus a device for mechanically eliminating the pressure is required. Thus, an attempt to eliminate localized pressure has been made using rupture discs. However, in the case of a large transformer, the arc generating point may be far from the rupture discs. Hence, before the pressure eliminating operation using the rupture discs is conducted, the tank may rupture. Further, the number of rupture discs is not sufficient compared to the arc energy, so that the tank may rupture before the pressure eliminating operation is performed.